The objective of this proposal is to develop a new class of selective miRNA regulators that blocks the maturation of oncomiRs. MicroRNAs (miRNAs) regulate diverse biological processes. Dysregulation of microRNAs (miRNAs) has become one of the emerging mechanisms contributes to cancer formation and progression. Increasing evidences suggest that certain miRNAs affect cellular transformation, carcinogenesis and metastasis, and act as oncogenes (oncogenic miRNAs or oncomiRs). Understanding the roles of oncomiRs in cancer will lead to the development of new cancer therapies. Besides, direct therapeutic intervention of oncomiRs has become a promising new direction for treating cancer. For both research and therapeutic purposes, methods allowing the control of oncomiR levels are highly desirable. Current methods for such purposes generally suffer from the off-target inhibition. To address this critical issue, we propose a novel strategy to regulate miRNA. We will develop a new class of bi-functional miRNA regulators consisting of independent and specialized pre-miRNA recognition unit and Dicer inhibition unit to target unique pre- miRNAs and block their processing to generate mature miRNAs by inhibiting Dicer enzymatic activity. The following Aims will accomplish our goal of establishing this new strategy: Aim 1. Synthesize and test bi- functional miRNA regulators. We will synthesize bi-functional regulators for miRNA-21 (miR-21) and other oncomiRs and test their inhibitory activity. Aim 2: Test the activity and specificity of the bi-functional miRNA regulator in cells. We will optimize cellular uptake of the regulator in mammalian cells. The cellular activity and off-target effects of the regulator will be evaluated in cells. This work will establish the technical foundation of a novel oncomiR regulation strategy. This unique technology will address the major obstacle encountered in current miRNA regulating methods and enhance the capability to dissect the roles of oncomiRs in cancer biology. It will significantly contribute to the advancement in our knowledge of the role played by oncomiRs in cancer and will result in the development of novel cancer therapies either directly based on the bi- modular regulators or evolved from the understanding of oncomiR functions unveiled by this new technology.